The present invention relates to a drive circuit of a capacitive load such as an EL (electroluminescence) element or a piezoelectric oscillator, and more particularly, to a drive circuit of a capacitive load or an integrated circuit suitable for driving a plurality of EL elements.
Conventionally, a drive circuit of a plurality of capacitive elements, for example, EL elements has the construction shown by FIG. 3. The structure is provided with a step-up circuit REG in which a coil L1 and a transistor Tr1 for driving the coil are connected in series between direct current power source terminals VDD and VSS. A surge pulse is generated at the coil L1 when the transistor Tr1 is switched on and off by a drive signal at a predetermined frequency. The surge pulse is applied to a high withstand voltage condenser C1 via a diode D1 to charge C1 by which high voltage is generated at a terminal CHV. Terminals on one side of EL elements E1 and E2, are connected with respectively exclusive constant current output circuits 31 and 32. Other terminals thereof are connected with a common constant current output circuit 33 and the constant current output circuits 31, 32 and 33 make respective EL elements emit light by using the high voltage from the step-up circuit REG. Further, an oscillator OSC generates the drive signal and a selector SEL2 controls the constant current circuits in correspondence with the EL element to emit light and applies bi-directionally output voltage to the EL elements.
Further, FIG. 4 shows another drive circuit for an EL element. In FIGS. 3 and 4 similar reference notations designate similar elements. In FIG. 4, the output voltage generated at the terminal CHV is compared with a predetermined voltage VREF by a comparator CP. A result of comparison is fed back to control the drive signal. By controlling power generated by the step-up circuit REG, the EL element is driven under constant voltage to adjust the brightness of the EL element. Further, switch circuits 41 and 42 are controlled by the selector SEL3 and bi-directionally apply output voltage respectively to the EL elements E1 and E2.
According to the drive circuit of the constant current drive system shown by FIG. 3, there is achieved an advantage that the capacitive load, for example, the EL element, does not to deteriorate or age since the capacitive element is driven by constant current. Since, power is supplied from the step-up circuit to the plurality of constant current output circuits, the respective constant current output circuits make the respective EL elements emit light, and power provided to all of the EL elements is constant. Accordingly, there poses a problem that respective brightnesses of the EL elements cannot be set individually.
Further, according to the drive circuit of the constant voltage drive system shown in FIG. 4, the output voltage to the EL element is compared with the predetermined voltage to control power generated by the step-up circuit. Accordingly, the EL element is deteriorated by ageing to reduce capacitance of the EL element. Therefore, the feedback control is operated in a direction of reducing power generated by the step-up circuit less than a value of initially using the drive circuit to reduce the brightness of the EL element.
It is an object of the invention to be capable of operating a capacitive load or adjusting respective brightness of, for example, an EL element in a drive circuit of a constant current drive system for making a plurality of capacitive loads, for example, EL elements emit light.
According to the invention, by setting a signal of driving a coil of a step-up circuit to a frequency in accordance with a driven capacitive load or a combination of the capacitive loads simultaneously driven, the output of the driven capacitive load can individually be set or the output can be set for a respective combination of the capacitive loads simultaneously driven. Thereby, for example, according to a drive circuit of EL elements for selectively driving a plurality of EL elements, respective brightness of an EL element driven by itself can be adjusted, and, brightness can be set for a respective combination of EL elements simultaneously driven.
According to an aspect of the invention, there is a drive circuit of a capacitive load comprising a step-up circuit in which a coil and a transistor are connected in series between a terminal at a first potential and a terminal at second potential lower than the first potential, a diode and a condenser are connected in series between a point of connecting the coil and the transistor and the terminal at the second potential and a surge pulse generated at the coil by switching on and off the transistor by a coil drive signal, is applied to the condenser via the diode to thereby charge the condenser by which output voltage is generated at a point of connecting the diode and the condenser, a high voltage switch output circuit for selectively driving a plurality of capacitive loads by constant current by receiving the output voltage from the step-up circuit, and a coil drive signal generating circuit for generating the coil drive signal at a frequency in correspondence with a single one of the capacitive load driven or a combination of the capacitive loads simultaneously driven by the high voltage switch output circuit.
According to another aspect of the invention, it is preferable that the coil drive signal generating circuit comprises a selecting circuit and a plurality of a oscillators, the plurality of oscillators are set to generate an oscillation output at a predetermined frequency in correspondence with either one of the plurality of capacitive loads and a combination of the capacitive loads, the selecting circuit selects the capacitive load to be driven and selects the oscillator(s) for generating a frequency in correspondence with the selected capacitive load or the combination of the capacitive loads simultaneously selected and the coil drive signal is generated based on the oscillation output from the selected oscillator.
According to another aspect of the invention, it is preferable that the capacitive load is an EL (electroluminescence) element and the coil drive signal is generated at a frequency for making the EL element(s) emit light with a predetermined brightness of one of the driven EL element or a combination of the EL elements simultaneously driven.
Further, it is preferable to provide an integrated circuit for driving a capacitive load capable.